Electronic cameras enjoy some important advantages over the traditional film camera because they store captured images in electronic form. Electronic storage is much less expensive than film; it is very high density, taking up much less space; it is non-fading, i.e., unlike film, its color information is substantially permanent; and it provides easy retrieval via computer. Electronic cameras of the current generation were designed primarily to have their captured images displayed on a television set. They are, therefore, made to be compatible with the National Standards Television Committee (NTSC) Standards. Among other things, the NTSC Standards specify interlaced fields of 2621/2 horizontal lines at 1/60 second intervals to produce complete frames containing 525 lines at a rate of 30 frames per second. Each field, therefore, contains every other line of the complete frame, and fields of odd lines alternate with fields of even lines.
Charge coupled device (CCD) sensors used by these cameras were designed specifically for this service. A typical device may have, for example, an array of 760 by 484 discrete photosensor sites, producing 760 by 484 pixel resolution and 484 horizontal lines. The device also provides a vertical shift register associated with each vertical column of photosensor sites, and the ability to simultaneously shift the pixel charges in alternate rows into their associated vertical registers. When the pixel charges in the vertical shift registers are sequentially shifted into a horizontal shift register for output, they produce a field containing every other line of pixel information. The following field contains the lines of pixel information omitted in the previous field. When the NTSC specified sync pulses are added, fields of 242 lines instead of 262 present no problem to the television set.
While the resulting 760.times.484 pixel resolution is quite acceptable for television viewing, it produces a print which is of poor quality compared to standard film cameras. There exists a need, particularly in commercial and industrial use, for a high-resolution, electronic cameras designed primarily for computer access to stored images and high quality prints. At least one new CCD photosensor has been developed to meet this need. The Kodak KAI-1000 interline CCD sensor has an array of 1024 by 1024 photosensor sites and 1024 vertical shift registers. For better computer access and printing, the sensor operates in the non-interlaced mode so that adjacent lines are stored in sequence. In order to provide a frame rate of 30 frames per second to capture motion and a reasonable read-out rate, the sensor has two horizontal shift registers. Two adjacent lines are read out simultaneously at a 20 MHz readout rate.
Along with the demand for the kind of performance this sensor offers is a need for image feedback so that accurate subject framing can be achieved. This is particularly true in the case of a camera for industrial or institutional use. The high resolution images produced by this type of sensor, however, cannot easily be displayed in real time. They require expensive wideband circuits and special high definition television techniques. An ordinary television type display, on the other hand, uses well known low cost circuits, but requires an interlaced signal.
It is desirable, therefore to have low cost apparatus and a method for producing a low resolution, real time image for a high resolution non-interlaced camera system.